If you mistyped your client's name, or want to change it for other reasons, then you click on the client's name and rewrite it.
You can also manage your client details from the option Details (in the action menu).
When the new window pops up, click on Settings; from here, you can personalize your client.
From here, you change the details for your client, such as:
Name
Title
Hourly Rate
Currency
Avatar text (Which letters should show in the circle)
And the avatar color (Can help you distinguish your clients more easily)